%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-3-21 on page I-347 in "paracol --- Several text streams aligned".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}

  \setlength\parindent{0pt}

  \setlength\marginparwidth{15pt}
  \setlength\marginparsep{5pt}
  \addtolength\textwidth{-2\marginparwidth} % make room on the page
  \addtolength\textwidth{-2\marginparsep}   % in the book example

%StartShownPreambleCommands
\usepackage{paracol} \marginparthreshold{2}
%StopShownPreambleCommands

\begin{document}
\begin{paracol}{3}
  Left\marginpar{L1}           \switchcolumn
  Middle \\ Middle \\
          Middle\marginpar{M2} \switchcolumn
  Right \\ Right\marginpar{R3} \switchcolumn*
\marginparthreshold{1}
  Left again\marginpar{L4}     \switchcolumn
  Middle\marginpar{M5}
\end{paracol}
\end{document}
